1. Field of the Invention
This invention relates to electroluminescent solid state devices, and more particularly, to an electroluminescent solid state device having a single crystalline metal oxide that is doped with one or more rare earth elements and an oxygen and/or fluorine activator atom.
2. Description of the Related Art
Solid state electroluminescent devices generally include a body of a single crystalline material that emits electromagnetic radiation when an electrical bias is placed across the body. The body generally includes means for causing the excitation of electrons in order to generate the radiation. Such means can include a PN junction within the body, or a metal insulator n type semiconductor (m-i-n) structure. The wavelength of the generated radiation is dependent on the composition of the material that comprises the body, including any dopants that are in the material.
Although early electroluminescent devices consisted of polycrystalline semiconductors, electroluminescent devices have been made from single crystalline semiconductor materials, particularly the group III-V compounds, and alloys thereof. It is known that doping a semiconductor with a rare earth element, such as erbium or terbium, provides a device that can generate radiation at a wavelength that is highly suitable for optical transmission purposes. However, it has been found that semiconductor devices that are doped with a rare earth element are not particularly efficient.
The concept of doping a transparent material with a rare earth element is known, for example, in U.S. Pat. No. 5,262,365 to Oyobe et al. This patent provides a silica-based glass that is doped with a rare earth element, aluminum, and fluorine. Oyobe seeks to avoid the crystallization problem that occurs when silica glass is co-doped with a rare earth element and aluminum. A SiO.sub.2 host glass is doped with the rare earth element erbium, or neodymium, aluminum, and fluorine. The glass composition of this patent is expressed by a glass matrix of a R.sub.2 O.sub.3 Al.sub.2 O.sub.3 SiO.sub.3 system, wherein R represents a rare earth element, and fluorine doping is conducted by substituting the oxygen of the system with fluorine.
The use of a doped crystal to form a laser is known. For example, U.S. Pat. No. 5,299,218 to Ban et al describes a blue light laser having the three embodiments shown in FIGS. 1-2, FIG. 3, and FIG. 4. In FIGS. 1-2, an active layer is activated by electrons that are emitted by tips that are within a field emission tip array. This array is energized by an electrostatic field application apparatus. The electrons pass through a vacuum space, and then impact the active layer. This active layer may be a doped alkali halide crystal, i.e. NaI doped with Tl (NaI:Tl), LiI:Eu, or CsI:Tl, or the active layer may be an un-doped alkali halide crystal, i.e. NaI, or the active layer may be an anthracene crystal, a trans-stilbene crystal, or the like. In the FIG. 3 embodiment, the active layer is a fiber that acts as a light waveguide. The remainder of the FIG. 3 device is the same as FIGS. 2, 3. This active layer is formed by crystal growth of anthracene. In the FIG. 4 embodiment, the active layer is an alkali halide doped, or undoped crystal, or an organic crystal and the remainder of the structure is the same as the FIGS. 1, 2 embodiment.
Also of general interest relative to wavelength tuning is U.S. Pat. No. 5,425,039 to Hsu et al wherein a tunable, single frequency, fiber optic laser is provided having an erbium:yttererbium phosphate glass fiber, or erbium:ytterbium phospho-silica glass fiber. A rare earth ion doped glass fiber is also mentioned. Wavelength tuning is achieved by temperature variation of laser gain cavity length, or by electromechanical variation of laser gain cavity length. Electromechanical tuning with a PZT transducer is also mentioned.
While prior devices have been generally useful for their limited intended purposes, it would be desirable to have an electroluminescent device that is doped with a rare earth element to achieve radiation at the desired wavelength, but which is formed of a material that provides greater efficiency to the device.